1. Field of the Invention
The present invention relates to a photovoltaic device and a method of manufacturing thereof and particularly relates to a photovoltaic device for converting light energy directly to electric energy and a method of manufacturing thereof.
2. Description of the Prior Art
A photovoltaic device for converting light energy directly to electric energy, that is, a so-called solar cell has attracted special interest in view of the problem of exhaustion of energy sources because the so-called solar cell uses mainly inexhaustible sunlight as an energy source. The sun gives an energy of approximately 1 kW/m.sup.2 to the surface of the earth when the weather is fair and assuming that the photoelectric conversion efficiency of a solar cell for converting light energy to electric energy is 10% for example, electric power of only 1 W is generated if a solar cell having an effective light receiving area of 10 cm.times.10 cm is used, and accordingly, a large area solar cell is required.
In order to satisfy such requirement, there has been developed a solar cell as disclosed in U.S. Pat. No. 4,064,521 in which an amorphous silicon film obtained by a plasma CVD method using a silicon compound gas as a raw material gas functions as a photoactive layer for performing photoelectric conversion. More specifically, an amorphous silicon film can be obtained easily by plasma decomposition of a silicon compound gas a a raw material gas and therefore such an amorphous silicon film is suited for making a solar cell have a large light receiving area. Furthermore, a photo CVD method using a low-pressure mercury lamp has also been utilized recently to develop an amorphous silicon solar cell.
In such a large area solar cell obtained by the plasma CVD method or the photo CVD method, a photoactive layer is in the form of a film and accordingly it is indispensably necessary to provide a substrate for supporting the photoactive layer. However, if the support surface of the substrate is a curved surface, it is difficult to obtain a photoactive layer having a uniform thickness and the optimum thickness for maximum photoelectric conversion efficiency cannot be obtained. In addition, even if a photoactive layer having a uniform thickness can be formed, it is extremely difficult in a photovoltaic device comprising such a photoactive layer to apply a desired fine patterning to an electrode film which electrically connect in series a plurality of photoelectric conversion elements as disclosed in U.S. Pat. No. 4,281,208, for example.
As a means for solving such difficulties, the prior art described in Japanese Utility Model Laying-Open Gazette No. 52640/1985 proposes a method in which a plurality of photoelectric conversion elements in the form of a film are formed in advance on a flat flexible substrate (a metal film of stainless steel, aluminium or the like) and the flexible substrate is adhered to the curved surface of the support plate making use of flexibility of the substrate.
However, if a flexible substrate is used as a supporting substrate for photoelectric conversion elements, its flexibility exerts, to the contrary, unfavorable influence on the manufacturing process, particularly the heating process, in which the final surface may be slightly curled due to a difference in the thermal expansion coefficient between the film of the photoelectric conversion elements and the flexible substrate formed on a flat surface or the photoelectric conversion elements might be broken because the flexible substrate is easily deformed by external pressure. In addition, if a photoactive layer is formed directly on the surface of a metal film, the photoelectric cells would be short-circuited, which would make it impossible to connect the photoelectric cells, and therefore, it is necessary to coat the surface of the metal film with an insulating film. However, if the thickness of the insulating film is not uniform or the insulating film contains pinholes or the like, the thickness of the photoactive layer also lacks uniformity and the photoelectric cells on the pinholes are short-circuited through the metal film, which causes the photoelectric conversion efficiency to be lowered and makes it impossible to connect the photoelectric cells. For the above described reasons, an insulating film is required to be formed in a manner in which it has a uniform thickness and does not contain pinholes, and accordingly, the manufacturing process becomes further complicated. Thus, a conventional photovoltaic device using a flexible substrate as a supporting substrate involves problems that the photoelectric conversion efficiency and other characteristics are not good in spite of a high manufacturing cost due to complication of the manufacturing process. Moreover, it is not suited for outdoor use, namely, it does not have good resistance to atmospheric conditions. Therefore, such a photovoltaic device cannot be put into practical use.